In recent years, sensor networks have been more and more used for the purpose of monitoring various types of information for managing residential environment, nature conservation, healthcare, traffic conditions, etc. In a sensor network, sensors having a communication function (may be called “sensor terminals” hereinafter) are disposed at various places, and the sensor terminals autonomously constitute the network. Each of the sensor terminals wirelessly communicates with surrounding sensor terminals using a random access method. A carrier sense multiple access/collision avoidance (CSMA/CA) method is an example method for the random access communication. Each of the sensor terminals identifies a terminal to which data is to be sent (that is, a relay terminal) among the surrounding sensor terminals according to a certain method for selecting a communication route, and thus selects a communication route. The sensor terminals in the sensor network constitute a wireless multi-hop network in this manner. The wireless multi-hop network is a wireless network in which each node has a function to transfer data, and nodes that are impossible to directly communicate with each other can communicate via a relay node. Each of the sensor terminals sends data to a destination device using multiple hops. The wireless multi-hop network is sometimes called an ad-hoc network.
Here, in the CSMA/CA method, before starting communication, each of the nodes performs carrier sensing to monitor a received signal strength indication (RSSI). When the detected RSSI is less than a certain value, or the RSSI is not detected, each of the nodes determines that no other node is sending data, and thus sends data. However, the CSMA/CA method has a problem of collision that occurs between a signal sent from a sending node to a destination node and a signal sent from a node that is in an area directly communicable with the destination node but is impossible to detect the transmission by the sending node. The collision may be called interference. The node in which a power of received signal transmitted from the sending node is less than a threshold value may be called a “hidden node”.
The wireless multi-hop network includes a tree-type network and a mesh-type network. The tree-type network includes a plurality of terminals and a gateway (GW) that is an information gathering station bundling the terminals. Existence of terminals at a high density under one gateway can produce traffic that is not accommodated by one gateway. In this case, additional installation of a new other gateway can increase the amount of traffic accommodated as a system, that is, the traffic capacity of the system.
However, when the newly added gateway is installed in an area from which an electromagnetic wave reaches the existing gateway, both of the gateways become competitors trying to acquire a wireless resource according to the CSMA/CA method. Specifically, while one of the gateways performs communication, the other gateway waits, and, as a result, is impossible to receive data directed to the other gateway itself, until the communication is terminated. Therefore, in such a case, adding the new gateway does not result in increasing the traffic capacity of the overall system.
There are conventionally methods to control transmission power of a plurality of terminals in the mesh-type network so as to suppress the interference between signals sent from the terminals. Conventional examples are described in Japanese Laid-open Patent Publication No. 2011-146850 and Japanese Laid-open Patent Publication No. 2011-254191.
However, simple application of the conventional interference suppression methods to the tree-type network causes a gateway to give a higher priority to quality of communication with a terminal directly communicating with the gateway itself than to suppression of the interference to other gateways through reduction in the transmission power. Therefore, the gateway giving interference to the other gateways reduces the transmission power thereof only to an extent that does not degrade the quality of communication with the terminal directly communicating with the gateway itself. As a result, the interference between signals sent from the gateways is not suppressed.
When the gateway simply reduces the transmission power to suppress the interference between the signals sent from the gateways, the communication with the terminal directly communicating with the gateway can be disconnected.